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Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies.

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Chapter 5 the data link layer

Our goals:

understand principles behind data link layer services:

error detection, correction

sharing a broadcast channel: multiple access

link layer addressing

reliable data transfer, flow control: done!

instantiation and implementation of various link layer technologies

Chapter 5: The Data Link Layer

5: DataLink Layer


Link layer

5.1 Introduction and services

5.2 Error detection and correction

5.3Multiple access protocols

5.4 Link-layer Addressing

5.5 Ethernet

5.6 Link-layer switches

5.7 PPP

5.8 Link virtualization: ATM, MPLS

Link Layer

5: DataLink Layer


Link layer introduction

Some terminology:

hosts and routers are nodes

communication channels that connect adjacent nodes along communication path are links

wired links

wireless links

LANs

layer-2 packet is a frame,encapsulates datagram

Link Layer: Introduction

data-link layer has responsibility of

transferring datagram from one node

to adjacent node over a link

5: DataLink Layer


Link layer context

datagram transferred by different link protocols over different links:

e.g., Ethernet on first link, PPP on intermediate links, 802.11 on last link

each link protocol provides different services

e.g., may or may not provide reliable data transfer over link

transportation analogy

trip from Princeton to Lausanne

limo: Princeton to JFK

plane: JFK to Geneva

train: Geneva to Lausanne

tourist = datagram

transport segment = communication link

transportation mode = link layer protocol

travel agent = routing algorithm

Link layer: context

5: DataLink Layer


Link layer services
Link Layer Services different links:

  • framing, link access:

    • encapsulate datagram into frame, adding header, trailer

    • channel access if shared medium

    • “MAC” addresses used in frame headers to identify source, dest

      • different from IP address!

  • reliable delivery between adjacent nodes

    • we learned how to do this already (chapter 3)!

    • seldom used on low bit-error link (fiber, some twisted pair)

    • wireless links: high error rates

      • Q: why both link-level and end-end reliability?

5: DataLink Layer


Link layer services more
Link Layer Services (more) different links:

  • flow control:

    • pacing between adjacent sending and receiving nodes

  • error detection:

    • errors caused by signal attenuation, noise.

    • receiver detects presence of errors:

      • signals sender for retransmission or drops frame

  • error correction:

    • receiver identifies and corrects bit error(s) without resorting to retransmission

  • half-duplex and full-duplex

    • with half duplex, nodes at both ends of link can transmit, but not at same time

5: DataLink Layer


Where is the link layer implemented

in each and every host different links:

link layer implemented in “adaptor” (aka network interface card NIC)

Ethernet card, PCMCI card, 802.11 card

implements link, physical layer

attaches into host’s system buses

combination of hardware, software, firmware

application

transport

network

link

link

physical

Where is the link layer implemented?

host schematic

cpu

memory

host

bus

(e.g., PCI)

controller

physical

transmission

network adapter

card

5: DataLink Layer


Adaptors communicating

sending side: different links:

encapsulates datagram in frame

adds error checking bits, rdt, flow control, etc.

receiving side

looks for errors, rdt, flow control, etc

extracts datagram, passes to upper layer at receiving side

Adaptors Communicating

datagram

datagram

controller

controller

receiving host

sending host

datagram

frame

5: DataLink Layer


Link layer1

5.1 Introduction and services different links:

5.2 Error detection and correction

5.3Multiple access protocols

5.4 Link-layer Addressing

5.5 Ethernet

5.6 Link-layer switches

5.7 PPP

5.8 Link Virtualization: ATM. MPLS

Link Layer

5: DataLink Layer


Error detection
Error Detection different links:

  • EDC= Error Detection and Correction bits (redundancy)

  • D = Data protected by error checking, may include header fields

  • Error detection not 100% reliable!

    • protocol may miss some errors, but rarely

    • larger EDC field yields better detection and correction

otherwise

5: DataLink Layer


Parity checking
Parity Checking different links:

Two Dimensional Bit Parity:

Detect and correct single bit errors

Single Bit Parity:

Detect single bit errors

0

0

5: DataLink Layer


Internet checksum review

Sender: different links:

treat segment contents as sequence of 16-bit integers

checksum: addition (1’s complement sum) of segment contents

sender puts checksum value into UDP checksum field

Receiver:

compute checksum of received segment

check if computed checksum equals checksum field value:

NO - error detected

YES - no error detected. But maybe errors nonetheless?

Internet checksum (review)

Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)

5: DataLink Layer


Checksumming cyclic redundancy check
Checksumming: Cyclic Redundancy Check different links:

  • view data bits, D, as a binary number

  • choose r+1 bit pattern (generator), G

  • goal: choose r CRC bits, R, such that

    • <D,R> exactly divisible by G (modulo 2)

    • receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!

    • can detect all burst errors less than r+1 bits

  • widely used in practice (802.11 WiFi, ATM)

5: DataLink Layer


Crc example
CRC Example different links:

Want:

D.2r XOR R = nG

equivalently:

D.2r = nG XOR R

equivalently:

if we divide D.2r by G, want remainder R

D.2r

G

R = remainder[ ]

5: DataLink Layer


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